In sputtering, the surface of a target is bombarded with ions under vacuum conditions. The bombardment blasts material from the target surface, which can be deposited onto substrates provided for this purpose that are placed in the field of view of the target surface. The ions required for this are provided by means of plasma that is built up over the target surface. By applying a negative voltage to the target, the ions are accelerated toward it. The more ions flow per unit time, the higher the coating rates are achieved. The higher the voltage that is applied to the target, the higher the impact velocity of the ions against the target surface is, and the higher the energy of the sputtered material that is blasted from the target. A higher power input is therefore desirable. Dependencies between the degree of ionization of the sputtered material and the power density are also known to exist. These effects are used in the HIPIMS process.
The middle power densities that are applied to such a sputtering target are generally in the range from 5 W/cm2 to 30 W/cm2.
Sputtering, however, is a PVD coating method that is not very energy efficient. This means that a large part of the energy provided in the target is converted into heat and the target heats up. This heat must be carried off by means of a cooling process. There are various approaches to achieve this in the prior art, which will be briefly outlined below.
a) Directly Cooled Target
With a directly cooled target 1, as schematically depicted in FIG. 1, the power that has been converted to heat on the target surface 3 is conveyed in the target material 5 by thermal conduction to the target back 7. The cooling fluid 11 flowing in a water duct 9 can carry off the flow of heat in accordance with its thermal capacity and the flow conditions. There is a very good thermal contact between the target back 7 and the cooling fluid 11. In this case, however, it is necessary to fasten the target to the base body 15, e.g. by means of screws 13. In addition, a seal 17 must be provided, which seals the vacuum off from the cooling fluid 11, for example water. Supply lines 6 are also outlined depicted in FIG. 1. Otherwise, the drawing is only a schematic depiction. Other components, for example for vacuum production, insulation, and the supply and removal the cooling fluid are known to experts and are therefore not shown here.
This directly cooled target is in fact attractive due to its very good cooling capacity, but has significant disadvantages due to presence of the coolant/vacuum seal and the necessary breaking of the water/target bond when changing targets. There is thus the danger, for example, of generating cooling fluid leaks. This danger is particularly high when frequent changes of the target material are required.
b) Indirectly Cooled Target
With an indirectly cooled target, as shown in FIG. 2, the back 203 of the target 201 is fastened to a source holder 205 (e.g. by screws or clamps) and an intrinsically closed cooling plate 207 is integrated into the source holder 205. For example, the cooling plate 207 includes a cooling duct 209 through which coolant flows, whose moving fluid carries off the heat.
In this case, the cooling fluid duct is bordered by a solid, stationary cover. For the sake of cooling and electrical contact, the target is fastened to this cover, for example with screws at the circumference or possibly in the middle of the target. This method leads, among other things, to two problems:
The heat transmission is produced through the surface of the target back and the surface of the cooling plate. Without particular measures, these two surfaces constitute a boundary surface that deviates sharply from that of an ideal, smooth contact pair. Such a situation is shown in FIG. 3. The heat transmission in this case is sharply reduced and turns out to be pressure dependent. Contact pressure, however, can only be introduced by means of the fastening screws, for example; in other words, the heat transmission can only be improved locally.
This situation can be improved by providing a contact film between the two surfaces. This film can, for example, be made of indium, tin, or graphite. Due to their ductility, these films can compensate for irregularities between the target back and the surface of the cooling plate. In addition, the contact pressure can be exerted more evenly over the area.
A disadvantage of this method is that it is awkward and difficult to mount a contact film, particularly with vertically mounted targets. This is particularly relevant when it is necessary to change the target materials frequently. In the case of graphite films, the lateral thermal conductivity is in fact good, but the transverse thermal conductivity is poor. Graphite films must therefore on the one hand be thin so that their low transverse thermal conductivities do not hinder the cooling process. On the other hand, a certain film thickness is necessary in order to avoid damage to the film during installation. For this reason, graphite films with a thickness of no less than 0.5 mm are used.
There is thus a need for an improved cooling device for targets, which in particular improves the changing of the target material as compared to the devices known from the prior art.